The field of the invention is pore-forming synthetic resins from foamed or foamable beads. The invention is particularly concerned with fine particle expandable styrene polymers containing small concentrations of organic bromine compounds to reduce the mold dwell time.
The state of the art of foamed polystyrene may be ascertained by reference to the Kirk-Othmer, "Encyclopedia of Chemical Technology," 2nd Edition, Vol. 9 (1966) under the section entitled "Foamed Plastics," pages 847-884 and Vol. 19 (1969) under the section entitled "Styrene Plastics," pages 85-134, particularly pages 116-120, where polystyrene foams are disclosed and pages 120, 121 where prior art hexabromocyclodecane is disclosed and U.S. Pat. Nos. 3,389,097; 3,503,905; 3,682,844; 3,789,028; 3,972,843 and 4,020,022, the disclosures of which are incorporated herein.
A process which has proved particularly practical in industry prepares molded objects by expanding fine-particulate, foamable styrene polymers in molds as disclosed by H. L. von Cube & K. E. Pohl in "Die Technologie des schaeumbaren Polystyrols," 1965. According to this process, the fine-particulate styrene polymers are first heated by means of steam or hot gases to temperatures above their softening points, whereby foaming takes place into discrete particles. This procedure denoted as pre-foaming. The pre-foamed polystyrenes are then temporarily stored and later further expanded by additional steam heating in a pressure-resistant mold whereby the particles weld into one another to a molded body corresponding to the inside cavity of the mold. This second procedure is denoted as final foaming. The molded object, after final foaming, is cooled inside the mold until the inside temperature drops below the softening point. When the molded object is prematurely removed from the mold, the object deforms. As foam plastics are good insulators, relatively long cooling times are required to cool the mold. The time interval allowing the earliest removal of the molded object without deformation is ordinarily called the "minimum mold dwell time."
It is therefore obvious that the processors of expandable polystyrene desire short minimum dwell times for reasons of economy. Accordingly, there have been many attempts in the past to make fine-particulate, expandable styrene polymers with low minimum mold dwell times. The prior art reveals numerous approaches leading to polystyrene foams with low minimum mold dwell times.
It is recommended, for example, to use fatty acid derivatives in concentrations between 0.1 and 2% by weight referred to the styrene polymer in order to achieve a low minimum mold dwell time as disclosed in U.S. Pat. Nos. 3,389,097 and 3,789,028 and British Pat. No. 1,408,267.
It is furthermore known to add various kinds of rubber in concentrations between 0.1 and 2% by weight referred to the styrene polymer as disclosed in U.S. Pat. Nos. 3,682,844; 3,972,843 and 4,020,022, British Pat. No. 1,297,105 and Japanese Published Application No. 49 052 862.
According to U.S. Pat. No. 3,503,905 and French Pat. No. 15 30 701 and British Pat. No. 1,171,657, brominated compounds are added to the polymerization inputs so that the bromine compounds are homogeneously distributed.
All the prior art processes, however, suffer from the drawback that the substances used negatively affect other processing properties of the expandable styrene polymers. Fatty acids or fatty acid derivatives and rubbers must be added in so high a concentration to achieve an appreciable lowering of the minimum mold dwell time that the softening point of the expandable styrene polymer is markedly lowered. This results in drawbacks both in processing, for instance, adhesion of the pre-foam, and a decrease in the strength of the final product. Beyond that, the economy of the process is decreased due to the high costs of the additives.
Also, relatively high concentrations of between 0.02 and 0.5% by weight are required for the bromine compounds recommended for lowering the cooling time if such is actually to be achieved. Such additives too, however, result in adverse effects on the mechanical properties of the foam and in particular as regards dimensional stability with respect to heat. Further, bromine compounds are frequently injurious to health.
Accordingly, attempts have recurringly been undertaken in the past to make do without adding foreign substances. Thus, British Pat. No. 1,082,966 recommends that the beads be very quickly cooled prior to foaming. French Pat. No. 15 00 623 recommends intermediary storage of the pre-foam to achieve a decrease in pressure and thereby a low minimum mold dwell time. However, neither process was found actually practical as these steps failed to achieve sufficiently short cooling times.
West German Published Application No. 2,542,281, the disclosure of which is incorporated herein, discloses criteria for special bromine compounds suitable as additions to expandable styrene polymers where the bromine compounds are sensitive to hydrolysis, the sensitivity to hydrolysis being indicated by the amount of hydrogen bromide per equivalent of bromine per molecule split off in bidistilled water. This amount is defined to be between 2.2 and 30% by mole. It is found upon closer examination, however, that this material is inadequate and that the special bromine compounds are practically only slightly effective, if at all. This result is easily grasped by considering that constant division and flowing together of the organic phase in drop form takes place in styrene suspension polymerization as disclosed by W. P. Hohenstein and H. Mark, J. Polym. Sci. 1(b 2), p. 138, 1946. The hydrogen bromide generated in hydrolysis is constantly transformed from the organic into the aqueous phase, whereby a substantial part of the bromine compound in the styrene polymer cannot become effective. Furthermore, the waste water is contaminated by hydrogen bromide.